What is the first byte of a Class B network number?

The first byte of a class B network number is in the range 128-191. In the number 129.144.50.56, the first two bytes, 129.144, are assigned by the InterNIC, and makeup the network address. The last two bytes, 50.56, make up the host address, and are assigned at the discretion of the owner of the network number.
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https://docs.oracle.com/cd/E19455-01/806-0916/6ja85399v/index.html
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